www.nature.com/scientificreports OPEN High frequency oscillations in epileptic and non‑epileptic human hippocampus during a cognitive task Martin Pail 1*, Jan Cimbálník2, Robert Roman1,3, Pavel Daniel1,3, Daniel J. Shaw3,4, Jan Chrastina5 & Milan Brázdil1,3 Hippocampal high‑frequency electrographic activity (HFOs) represents one of the major discoveries not only in epilepsy research but also in cognitive science over the past few decades. A fundamental challenge, however, has been the fact that physiological HFOs associated with normal brain function overlap in frequency with pathological HFOs. We investigated the impact of a cognitive task on HFOs with the aim of improving diferentiation between epileptic and non‑epileptic hippocampi in humans. Hippocampal activity was recorded with depth electrodes in 15 patients with focal epilepsy during a resting period and subsequently during a cognitive task. HFOs in ripple and fast ripple frequency ranges were evaluated in both conditions, and their rate, spectral entropy, relative amplitude and duration were compared in epileptic and non‑epileptic hippocampi. The similarity of HFOs properties recorded at rest in epileptic and non‑epileptic hippocampi suggests that they cannot be used alone to distinguish between hippocampi. However, both ripples and fast ripples were observed with higher rates, higher relative amplitudes and longer durations at rest as well as during a cognitive task in epileptic compared with non‑epileptic hippocampi. Moreover, during a cognitive task, signifcant reductions of HFOs rates were found in epileptic hippocampi. These reductions were not observed in non‑epileptic hippocampi. Our results indicate that although both hippocampi generate HFOs with similar features that probably refect non‑pathological phenomena, it is possible to diferentiate between epileptic and non‑epileptic hippocampi using a simple odd‑ball task. Te discovery of high-frequency electrographic activity represents one of the essential milestones not only in epilepsy research, but also in cognitive science over the past few decades. Tese transient high and very high-frequency oscillations (HFOs/VHFOs) in invasive EEG (stereoelectroencephalography; SEEG) have been recorded repeatedly in several allocortical and neocortical structures. Tese short-lasting feld potentials, both ictal and interictal phenomena, can be divided further into “ripples” (80–250 Hz), “fast ripples” (250–600 Hz), “very fast ripples” (VFR; 600–1000 Hz), and “ultra-fast ripples” (UFR; 1–2 kHz), all of which have been studied widely in humans under physiological and pathological conditions 1–12. HFOs are believed to stem from the short-term synchronization of neuronal populations and their activity, and it appears that they are connected to normal as well as pathological brain functions6,13. While physiological HFOs seem to represent summated synchronous inhibitory postsynaptic potentials (IPSP) generated by interneu- ronal cell subpopulations regulating the principal cell activity and their discharges 14. Epileptic HFOs might refect the feld potentials which are formed by the activity from clusters of abnormal synchronously bursting pyramidal cells, generating population spikes, and decreased inhibitory interneuron fring6,15. Te detected HFO frequency can be determined purely from the behavior and activity of one cell subpopulation (“pure” HFOs). However, the observed HFOs (especially beyond the physiologic limits of neuronal fring; > 300 Hz), may also represent the net frequency of neuronal populations, more specifcally due to the activity of diferent cell subpopulations of 1First Department of Neurology, Brno Epilepsy Center (Full member of the ERN EpiCARE), St. Anne’s University Hospital and Medical Faculty of Masaryk University, Pekařská 53, Brno 65691, Czech Republic. 2International Clinical Research Center, St. Anne’s University Hospital, Brno, Czech Republic. 3CEITEC – Central European Institute of Technology, Masaryk University, Brno, Czech Republic. 4School of Life and Health Sciences, Aston University, Birmingham, UK. 5Department of Neurosurgery, Brno Epilepsy Center, St. Anne’s University Hospital and Medical Faculty of Masaryk University, Brno, Czech Republic. *email: [email protected] Scientifc Reports | (2020) 10:18147 | https://doi.org/10.1038/s41598-020-74306-3 1 Vol.:(0123456789) www.nature.com/scientificreports/ synchronized neurons between which is a phase delay in activity. Each cell assembly then usually oscillate with a lower frequency than observed (“emergent” HFOs)16,17. Tere is evidence that ripple generation is infuenced by larger networks; smaller networks are involved in fast ripples and even less in VFR and UFR generation, which can be observed focally9. Interest in HFOs is related primarily to the localization of the epileptogenic zone, since they are considered as being more focal and specifc than classical epileptic spikes18 that are only partially concordant with the epileptogenic zone19. As HFO rates are higher in focal seizure-generating tissue, they have attracted attention as a possible clinical biomarker 3. Unfortunately, pathological and physiological HFOs cannot be distinguished by rate of their occurrence, as some regions are identifed as generators of physiological HFOs12. Moreover, another signifcant problem, has been the fact that pathological HFOs overlap in frequency with physiological HFOs associated with normal brain function8,20–22. How these two types of electrographic phenomena can be separated remains unclear6, as frequency and/or amplitude analysis alone seem to be insufcient for their delineation23–25. Terefore, the translation of HFOs into clinical practice is hindered by the inability to diferentiate between pathological and normal HFOs in SEEG recordings. And so, more than twenty years afer their discovery, there still exist questions about HFOs as biomarkers of epileptogenic brains and epileptogenic zones, and about their utility in clinical practice 12,26,27. Until now, various analytical approaches have been sought to distinguish pathological and physiological HFOs. Te methods used most commonly are based on, for example, a specifc regional distribution in mesial temporal structures28; the association of HFOs with epileptiform discharges 4,26,29, slow waves30 o spindles31, or the diference between HFOs produced spontaneously and those induced by a cognitive task8,23,24. In the recent study of Sakuraba et al., epileptogenic region was determined based on less suppressive efect of REM sleep on HFOs in contrast to non-epileptogenic/physiological region 32. Some authors have suggested separating HFOs by clustering them based on features such as frequency, duration, and amplitude24,25,33. Other reports have addressed this problem proposing several methods for dissociating diferent origins based on the width of the spectral frequency content of individual events, number of distinct cycles observed, and the presence of actual oscillations in the unfltered raw signal34,35. Finally, when investigating electrophysiological brain recordings, the term HFO should be used to describe true high-frequency local feld potential oscillations in the invasive EEG, that is oscillations visible in the raw recording and not the high-frequency Fourier components from a bandpass flter36. Te ability to distinguish between pathological and physiological HFOs is crucial for understanding normal cognitive functions and no less important for the translation of HFOs into clinical practice. In a recent study, we tested the hypothesis whether the presumed efect of cognitive task on hippocampal ripples can be used as a new approach for distinguishing pathological HFOs in the epileptic hippocampus (EH) from physiological HFOs in the non-epileptic hippocampus (NEH)8. To diferentiate them, a simple oddball task was used. Tis study revealed diferent and, in some cases, opposing behavior of ripples within EH and NEH: Ripples were signifcantly more reduced during a cognitive task than in a resting period in EH, but in NEH this diference remained statistically marginal8. Moreover, we observed a signifcant suppression of ripple rate in the frst second afer stimulus onset only in NEH8. Importantly, however, we did not examine fast ripples due to a low sampling frequency. In the present study, we tested the hypothesis that not only ripples, but also fast ripples are modulated by cognitive tasks. We aimed to fnd a distinct impact of a cognitive task on HFOs (the rate and other HFO char- acteristics) within EH and NEH. To test the hypothesis, we analyzed hippocampal SEEG of 15 patients during resting period and during a simple cognitive oddball task. Methods Subjects. In our study we included 15 patients (7 females) ranging in age from 24 to 56 (mean: 38.3 ± 9.3) years. All patients sufered from medically intractable focal temporal epilepsies. For demographic and clinical characteristics of the included subjects, see Table 1. In most patients, chronic anticonvulsant medication was reduced slightly for the purposes of video-SEEG monitoring. Te study procedures were approved by Masaryk University and St. Anne’s University Hospital Ethics Committees. All subjects gave their written informed con- sent prior to the study investigation. All methods were performed in accordance with the relevant guidelines and regulations. EEG recordings. Patients underwent the implantation of depth electrodes as part of their evaluation for pharmacoresistant focal epilepsy in order to localize seizure origin prior to surgical